The Astronomy Book

64

See also: Elliptical orbits 50–55 ■ Halley’s comet 74–77

I

n 1639, a 20-year-old English

astronomer named Jeremiah

Horrocks predicted a transit

of Venus across the face of the sun

after finding errors in tables made by

Johannes Kepler. Because the transit

was only four weeks away, Horrocks

wrote to his collaborator, William

Crabtree, urging him to observe

it. On December 4, 1639, Horrocks

and Crabtree independently set up

helioscopes that focused an image

of the sun from a telescope onto a

plane. They became the first people

to witness a transit of Venus.

As it crossed the sun’s disk,

Horrocks tried to calculate Venus’s

size and distance. He noted that it

subtended, or spanned, an angle

of 76 arcseconds (^76 ⁄ 3600 °) at Earth

(p.58), which was smaller than the

value guessed by Kepler. Using the

ratios of planetary distances known

from Kepler’s third law, Horrocks

calculated that the disk of Venus

subtended an angle of about 28

arcseconds as seen from the sun.

Using data from a transit of

Mercury that had taken place in

1631, Horrocks calculated that

Mercury subtended the same angle

as Venus. He guessed that all the

planets subtend the same angle

at the sun, and calculated the

distance from Earth to the sun to

be 59 million miles (95 million km).

Horrocks’s guess is now known

to be wrong: Earth subtends 17.8

arcseconds at the sun, which is 93

million miles (150 million km) away.

Nevertheless, he was the first to

have a reasonably accurate idea

of the size of the solar system. ■

A PERFECTLY

CIRCULAR SPOT

ENTERED ON THE SUNC

THE TRANSIT OF VENUS

IN CONTEXT

KEY ASTRONOMER

Jeremiah Horrocks

(1618–1641)

BEFORE

c.150 ce Ptolemy estimates the

Earth–sun distance at 1,210

times Earth’s radius—around

5 million miles (8 million km).

1619 Kepler’s third law

gives the ratio of the sizes

of planetary orbits but the

absolute values are not known.

1631 French astronomer Pierre

Gassendi observes a transit of

Mercury across the solar disk,

the first planetary transit to be

recorded in history.

AFTER

1716 Edmond Halley

suggests that an accurate

timing of the transit of Venus

could lead to an accurate

Earth–sun distance.

2012 The most recent transit

of Venus takes place. The next

two will be in 2117 and 2125.

The most recent transit of Venus

in 2012 (the tiny dot in the top right

of the sun’s disk) was captured by

NASA’s Solar Dynamics Observatory.